1. Field of the Invention
The present invention relates to an apparatus and method for measuring the stress in an elongate steel member and, specifically, without limitation, for dynamically measuring the longitudinal stress in each rail of a railroad track having continuous rails.
2. Description of the Related Art
In the past, railroad tracks have largely been constructed by anchoring discrete rail sections having finite length to an underlying road bed in an end-to-end relationship. A distinct advantage provided by such an arrangement is that differences in thermal coefficient of expansion between the metal rail sections and the road bed is absorbed by mechanical joints or gaps, which are purposefully provided between adjacently spaced sections as the rail sections are installed. Unfortunately, those gaps cause substantial maintenance problems, which generally require lifting and aligning selected ones of the rail sections, reconstruction of the sub-grade or road bed, and the like. Besides added maintenance problems, the mechanical joints also cause ride, noise and comfort problems.
During the last three decades, an effort has been underway to eliminate the mechanical joints in railroad tracks. That effort has largely involved constructing tracks having continuous rails by welding or otherwise joining together the ends of the adjacently spaced rail sections, forming a structure sometimes referred to as continuous rail track, or "CR" track. The technology associated with the construction of a CR track is well known in the prior art.
The elimination of mechanical joints from rail tracks by joining together the rail section ends, however, creates new problems arising from seasonal variations in the ambient temperature. In tropical climates, the ranges between the temperature extremes are generally moderate, which does not pose a substantial problem for rail systems. In temperate climates, however, such as those in the United States, Asia, Australia and Europe, the ranges of temperature extremes are sufficient to cause catastrophic, temperature induced failures in rail systems, including both rail pull-apart and rail-buckle failures, as hereinafter described.
For example, an unanchored 100-mile length of continuous rail in certain areas of a temperate climate could experience a change in length of over 700 feet from one seasonal temperature extreme to the other. By anchoring the rail to railroad ties, changes in the overall length of the rail can be largely prevented but, instead, resultant localized longitudinal stresses are created internally in the rail.
As the rails of a CR track are initially installed and anchored to a road bed, each of the rails has zero longitudinal stresses. The temperature at which the CR track is installed is sometimes referred to as the stress-free temperature, or "SFT".
As the ambient temperature falls below the SFT, tensile longitudinal stresses are created internally in each rail of the CR track due to the greater thermal coefficient of expansion of the metal rails relative to that of the underlying road bed. If the difference between the reduced ambient temperature and the SFT is extreme, the tensile stresses in the rails can potentially attain sufficient magnitude to actually cause one or both of the rails to pull apart.
Preferably, a CR track is installed at a temperature such that the magnitude of the maximum, cold-temperature induced, tensile stresses are substantially less than those required to produce a rail pull-apart. If one or both of the rails of a CR track should pull apart in extremely cold conditions, risk exposure for derailment is generally minimal as pull-apart failure usually occurs beneath a moving train and the resulting separation between the two ends at the pull apart may not be sufficient to cause derailment. Failure to anchor the track at an appropriate SFT, however, may result in much larger separations--gaps of up to 6 inches have been recorded--which do pose a serious threat of derailment.
Fortunately, pull-apart failure can easily be detected by establishing an electrical track circuit using the rails as part of the conduction path, which becomes "open" if one of the rails of the CR track pulls apart.
Similarly, as the ambient temperature climbs above the SFT, compressive stresses are created internally in each of the rails of the CR track. If the difference between the elevated ambient temperature and the SFT is extreme, the compressive stresses in the rails can potentially attain sufficient magnitude to actually cause one or both of the rails to buckle. Theoretical calculations indicate that a stress of approximately 200 Kips (kilopounds) is sufficient to cause rail buckle during extreme Summer temperatures. (Strictly speaking, stress is a measure of force per unit area; however, since the cross-sectional area of a given rail is substantially constant, the expression herein of longitudinal rail stress in force units should be interpreted to mean force per cross-sectional area of the rail.)
Such buckling, which is random, unpredictable and a major source of derailments, generally occurs as a result of the CR track being anchored at a temperature whereby excessive compressive stresses are generated in the rails during peak Summer temperatures. The ability of a train to negotiate a lateral rail displacement, which is typical of rail-buckle, is minimal. As a result, rail-buckle poses a substantially greater risk of derailment than does a rail pull-apart since the former cannot be detected by a conventional track circuit.
To reduce the risk of failures occurring during both upper and lower temperature extremes, the rails of a CR track are generally anchored to the underlying road bed at a preferred SFT, or "PSFT" which is generally situated approximately mid-way between the upper and lower extreme temperatures normally realized for each locality containing the rails. The PSFT is generally defined as the CR installation temperature at which the CR track, hopefully, will not fail due to rail pull-aparts during extremely cold ambient temperatures, nor fail due to rail-buckle during extremely hot ambient temperatures.
The PSFT for any particular locality should take into account several variables, including temperature extremes, rail size and cross-section, track structure design, curves and tangent lengths, rail anchor design, tie type and weight, number of anchors per tie, track geometry and profile, ballast modulus, etc. If anchoring of the continuous rails occurs at temperatures substantially removed from the PSFT for a particular locality, the risk that the track may fail either from rail pull-apart or from rail-buckle is substantially enhanced.
For example, in one locality in Wyoming, the rails of a CR track are generally constructed of 136 lb./yd. rail stock and are generally anchored at a PSFT of approximately 95.degree. F. (In a 136-lb./yd. rail, industry standards indicate that the longitudinal stresses change by approximately 1.8 Kips/.degree. F.) In Wyoming, rail temperature extremes ranging from -50.degree. F. to 160.degree. F. have been recorded. Therefore, rail-buckle can occur if a CR track is anchored at ambient temperatures substantially below the PSFT of 95.degree. F.
In one instance during extreme Winter conditions, a rail of a CR track pulled apart with the width of the resulting gap between the two rail segments indicating that a stress of approximately 300 tons/inch.sup.2 had caused the failure. If the rail had been properly installed at the PSFT of 95.degree. F., then an ambient temperature of -50.degree. F., or temperature differential of 145.degree. F. from the PSFT, would have been insufficient to develop the indicated catastrophic tensile stress.
Even if a CR track is anchored at the PSFT, the desired stress characteristics arising therefrom are compromised each time a railroad maintenance crew routinely repairs fatigue defects in the track, which repairs occur with a frequency of approximately 200,000 annually within North America alone. The inability to re-establish the PSFT characteristics at each site after such repair results in previously unascertainable, cumulative deviations of the actual longitudinal rail stresses of the repaired rail from those which preferably would exist relevant to the PSFT. Destruction of the preferred internal stressing characteristics is accentuated for repairs which are performed during extreme temperature conditions.
For example, the installation of a rail insert, sometimes referred to as a "plug", in extremely cold ambient temperatures far below the PSFT can result in relieving the existing tensile stresses which would otherwise have countered a certain portion of the compressive stresses anticipated to arise during the hot Summer season.
Similarly, the opposite effect is realized if a stress-free plug is inserted in extremely hot ambient temperature, far above the PSFT, which then substantially aggravates the excessive, relatively localized tensile stresses occurring during the cold Winter season.
The seasonality of the longitudinal stresses, complicated by the continuity interruptions, resulting from maintenance and repair of rail fatigue and other defects, is believed to cause the existing longitudinal rail stresses to be largely unknown, thereby making predictability of rail-buckle failures virtually impossible.
Even if the rails of a CR track are installed at or near the PSFT, each of the rails and the road bed are dynamic systems. Thus, absolute longitudinal stresses in the rails need to be periodically monitored and controlled.
The economic consequences of derailments is substantial. In 1989, the FRA reported that within the United States alone, rail induced derailments caused damages of approximately $56,000,000. Although not all of those derailments arose from a single type of cause, rail-buckles were responsible for a significant portion of those damages, not to mention the concurrent loss of life.
Although various methods and apparatus have been developed in an attempt to prevent rail-buckle in a CR track, none of them are capable of dynamically, accurately, and non-destructively measuring the absolute longitudinal stress in the rails of a CR track. For example, traditional methods for determining longitudinal stresses in a rail of a CR track include severing the rail, observing the resultant gap or closure between the severed ends, and analyzing the width of the gap or closure as a function of the ambient temperature. Obviously, this method is a destructive and undesirable procedure.
Fortunately, the propagation velocity of ultrasound through a continuous rail varies as a function of the magnitude of the internal stresses of the rail. Unfortunately, however, other methods and apparatus utilizing ultrasonic techniques for detecting various types of flaws and defects in rails have suffered from interference arising from flange noise. Flange noise is generated as the flanges of the wheels of a deployable vehicle containing the apparatus roll along the rails such that multi-point contact is made between the rails and the wheels at different rolling diameters, thereby causing a situation sometimes referred to as "slip".
The sound spectrum generated by slip generally spans a particular ultrasonic frequency, such as 2.25 MHz., which could be usefully employed to measure longitudinal stresses. Without filtering or otherwise minimizing the effects of the flange noise, the randomness and magnitude of this interference is sufficient to mask the useful data which could otherwise be obtained by ultrasonic techniques.
It has long been recognized that the time of flight of an ultrasonic signal through a rail varies as a function of the longitudinal stress in the rail. However, with chemistry and metallurgical effects also changing the flight time, it has been virtually impossible to isolate the flight time change which is directly attributable to longitudinal stress. Accordingly, other, more reliably measurable relationships between ultrasonic transmission characteristics and longitudinal stress must be utilized.
What is needed is an apparatus and a method which can be used to minimize or eliminate the effects of sonic interference from flange noise and which, therefore, can be used to dynamically and non-destructively measure the absolute longitudinal stresses in the rails of a CR track. Such an apparatus and method must use ultrasound characteristics other than a straight time of flight measurement, since no technique has been created to measure a reliable, isolatable longitudinal stress effect using this variable.